The present invention relates to an electro-optical or single carrier electronic device that utilises the simultaneous application of two non-parallel electric fields.
Opto-electronic devices are well known. They use a means responsive to light to generate a photo-current, a structure that has a semiconductor quantum well region, and a means that responds to the photo-current so as to electrically control an optical absorption of the semiconductor quantum well region. The optical absorption of the semiconductor quantum well region can vary in response to the variations in photo-current.
Conventional modulator and detection devices employ schemes that use reversed biased diodes containing not intentionally doped (NID) optically active regions sandwiched between conductive layers of p-doped and n-doped semiconductor layers. A photo-current generated will be superimposed upon the dark current of the reversed biased diode. Non-linear electro-optic effect and bistable devices are therefore essentially a two electrical port and two optical port (input and output) device. The photo-current that is generated using the optical non-linearity of multiple quantum structures inside the active region can be used in an external circuit to provide feedback to the device itself. This is commonly referred to as the self electro-optic effect device (SEED). The applied electric field modulates the absorption (excitonic in nature due to the reduced dimensionality of the quantum well) of the active layer by the use of the quantum confined Stark effect. The present invention seeks to improve on known devices by separating the photo-current from the perpendicular biased electric field so as to produce a four electrical port device, by simultaneously applying non-parallel fields. In an optimum configuration one applies both perpendicular and parallel electric fields (that may be intrinsically or externally applied) to a material containing two-dimensional quantum wells (or superlattice) one dimensional quantum wires or zero dimensional quantum dots (comprising the active regions).
The device predominantly consists of layered dissimilar semiconductor materials that comprise of combinations of bulk material and ultrathin layers utilising the quantum size effect. The device is structured so as to contain an optically active region such that the optical and/or electrical response can be altered by the application of externally applied electric fields. Depending up the type of active region, there exists spatial directions mostly perpendicular and parallel which can be used to either extract or inject current carriers (intrinsic to or photogenerated within the material) or modify built in electric potentials within the said active region.
The present invention is for devices that can have wave guide and grating implementations for coupling the input optical signal into the active region and coupling of the processed optical output from the device.
Using quantum well material, the modulation is accomplished by an electric field parallel to the growth direction (perpendicular to the plane of the semiconductor layers). The electric field is generated by two n-doped layers sandwiching the active regions. Electrical contacts are via OHMIC metallisation to the n-doped layer. Vertical transport (perpendicular to the plane of the layers) is inhibited through the active region by specially designed blocking layers for electrons and/or holes. These blocking layers are used to control the dark current of the device. A photo-current generated in the active region is therefore also inhibited from reaching these contacts. A simultaneously parallel applied electric field, which can be used to extract/inject current carriers is realised by electrically contacting the layers in a spatial direction parallel to plane of the quantum wells.
The single dopant species is especially useful for integration with high electron mobility transistors so as to be compatible with monolithic microwave integrated circuits processing. The device described may also manifest as an optical active region comprising of a single or multiple two-dimensional electron gas modulation doped hereto structure. The control of electron (or hole for the case of two-dimensional hole gas) concentration in the active region can also be used for modulation of both the optical and electrical properties.
The use of strain layer material comprising the quantum well region can be used to deform the in-plane energy dispersion (hole energy versus in-plane wave vector relationship) of the valence band so as to dramatically reduce the in-plane effective mass of the quantised hole subbands. This allows the device to be optimised for high speed applications requiring the fast extraction of holes from the active region. This concept is intrinsic to development of the dual field device described herein.
The invention can therefore be used as an electro-optic device suitable for the modulation of an optical signal (modulator) or detection of an optical signal (receiver). The unique feature is the separation of the photo-current generated within the optically receptive region of the device and the modulating field necessary for altering the absorption profile of the active region. Devices using a single carrier species (such as electrons or holes only, employ the same mechanism.
Optical radiation that is incident upon the device is coupled into the active region using two different configurations. Firstly, one can use a wave guide type geometry that is an optically guided wave mode mostly in the plane of the quantum well layers. The guided mode is at right angles to both the parallel and perpendicular electric fields. Second, using optical coupling mostly parallel to the growth direction, the guided wave has a propagation vector colinear with the perpendicular electric field and at right angles to the parallel field. The optically active material is aligned to the respective fields required for the desired extraction and modulation effects.
The present invention is preferably for an active region comprising of multiple quantum wells using AlGaAs semiconductors. The electric fields are established via a single n-type dopant species in an attempt to make the said device compatible with monolithic microwave integrated circuits processing suitable for high frequency implementations. Other implementations such as using p-type only or conventional methods of generating the required electric fields by using p-type and n-type layers to form p-i-n diodes are also possible. It should also be noted that hetero structure material systems from other III-V (e.g. InGaAsAIP), II-IV (CdZnTe), IV-IV (SiGe) or amorphous silicon compounds may equally be applied to this technology.
Therefore in one form of the invention though this need not be the only or indeed the broadest form there is proposed an optical device including;
means responsive to light for generating a photo-current including;
a structure having a semiconductor quantum well region, and a means for the simultaneous application of non-parallel electric fields to said quantum well structure.
In preference, the non-parallel electric fields are perpendicular to each other.
In preference, one electric field functions in the electronic transport for the extraction injection of electrons and/or holes perpendicular to the quantum well direction via a parallel electric field directed with the vector lying in the plane of the quantum well layers.
In preference, the other electric field is a perpendicular electric field with the vector parallel to the growth axis and is used for modulation of absorption profile of the quantum well structure.
In preference, the perpendicular electric field is generated by single broken species.
In preference, a dark current perpendicular to the layers is controlled by barrier layers disposed at either side of the active region and further sandwiched between the said n-type layers.
The barrier layers thus provide a high energy potential to the electrons (and holes) along the growth direction. The barrier may be either bulk or superlattice material allowing the use of n-type dopants if p-type dopants are not possible.
In preference, the device includes strained layer quantum well material within the active region grown on crystallographic orientations. This enhances the piezo electric coefficient of the material since one can generate a piezo electric field with vector lying either at parallel or perpendicular to the growth directions. A device which has a piezo electric field lying within the plane can be externally modulated via the parallel electric field contact arrangement whilst simultaneously using the QCSE to modulate the energy bands quantised along the growth direction.
With such a device with a piezo electric vector lying in parallel to the growth direction of strained quantum wells also allows the operation of dual field devices which can modulate the exciton energy so as to produce blueshifting modulator/detector.
By controlling the extraction of photo carriers parallel to the quantum wells, the optical response of the device can be varied. That is, by reducing the extraction efficiency of photo generated charge in the active region, electrons in holes will accumulate especially opposite regions of the barrier material-active layer interface (for non zero electric field power to the growth direction). The space charge generated in this way can reduce the electric field parallel to the growth direction and thereby provide a switching function.
By injecting charge carriers into the device by either allowing tunnelling of thermionic emissions of carriers from the dot regions or via the n-plane contacts, a device can be made to alter its absorption properties by phase space filling effects. This may be able to switch the exciton resonance off due to the population of quantised energy levels by electrically injected carriers.
Such an optical modulation enhanced device can be fabricated in AlGaAs and can act as a modulator or a receiver. The device can further be suitable for plane integration.
By utilising strained layer quantum well material for the active region, the electronic structure of the valence band can be dramatically altered. The n-plane dispersion of the valence band of quantum well structures can be warped so as to decrease the n-plane heavy hole effective mass close to the brillouin zone centre for zinc blend host layer materials. The capability of extracting of these modified heavy-holes in the plane of the quantum well allows the device to operate in high speeds.
The electro spacing for the electrical contacting of the dual field device can enhance its operation as a transmission and reflection optical grating elements. The intensity of the transmitted and reflected diffraction orders are controlled by the electro optic parameters of the active layer.
The single dopant structure may not need to contain the depletion region to function. That is, devices such as hetero junction bipolar transistors require the base to be doped opposite to the emitter and the collector regions. Similarly field effect resistors such as MESFET and HENT require the use of Schottky gate contacts to alter the conducting channel properties. The present device therefore may not have a built in electric field across the active region when an external device is not present.